Focusing of short acoustic pulses is a fundamental aspect of most of the medical and industrial applications of ultrasound. In medical imaging, the quality of focusing of acoustic waves is directly related to such an important parameter of imaging as its spatial resolution. In therapeutic and surgical applications, effective focusing of ultrasound is important for delivering sufficient amount of acoustic energy to a target tissue to achieve necessary biological effect as well as for selective action on the lesion which needs to be treated without damaging surrounding healthy tissues. The use of focused ultrasound in medicine is described in detail in various patents as listed below (these patents are all incorporated herein in their entirety by reference): U.S. Pat. Nos. 5,207,214; 5,590,657; 5,613,940; 5,762,066; 5,769,790; 6,128,958; and 6,488,630.
Conventional approaches to focusing acoustic waves include geometrical focusing and electronic focusing. Geometrical focusing is based on the use of a transducer manufactured as a spherical shell or acoustic lenses (O'Neil, H. T. “Theory of focusing radiators”. J. Acoust. Soc. Am. 1949, 21, pp. 516-526). Geometrical focusing systems are simple, inexpensive and easy to make, but their principal disadvantage is that they have the fixed focal distance and could not steer the focus along and off the axis. The electronic focusing is based on the use of phased-array systems consisting of a number of separate elements (Ebbini E. S., Cain C. A. “A spherical-section ultrasound phased-array applicator for deep localized hyperthermia”. IEEE Trans. Biomed. Eng. 1991 V. 38. No. 7. pp. 634-643). Each element is excited by its own circuit that allows changing in a controllable way of the phase relationships over the array aperture, therefore creating any desired shape of a wave front.
An alternative technique of focusing ultrasonic waves is based on principles of Time-Reversed Acoustics (TRA) which were developed by M. Fink (“Time Reversed Acoustics”, Physics Today, March 1997, pp. 34-40, and Fink M. “Time reversed acoustics”. Scientific American, pp. 91-97 (1999), both of these articles are incorporated herein by reference). The TRA technique is based on the reciprocity of acoustic propagation, which implies that the time-reversed version of an incident pressure field naturally refocuses on its source. U.S. Pat. No. 5,092,336 to Fink, which is also incorporated herein by reference, describes a TRA device for focusing of acoustic waves in tissues.
Several practical examples of TRA ultrasound focusing system are described in the U.S. patent application Ser. No. 10/370,134 (U.S. Patent Application Publication No. 2004/0162550), now U.S. Pat. No. 7,201,749, and U.S. patent application Ser. No. 10/370,381 (U.S. Patent Application Publication No. 2004/0162507), now abandoned, to Govari et al. as well as his counterpart European Patent Application No. EP 1449564, all of which are incorporated herein by reference.
The bandwidth of the probes generating ultrasound is mainly defined by the bandwidth of the transducers employed in the probe. These transducers are typically made in the form of discs and plates of piezoelectric materials of certain constant thickness, which defines resonance frequencies of the transducer. Ultrasound is radiated near resonance frequencies much more efficiently than at other frequencies. Use of resonant transducers limits the possibility to generate short pulses since the shorter is the pulse the wider is its spectrum. The pulse cannot be made much shorter than the ringing time of the transducer. Transducers resonance frequencies dominate in the spectrum of the focused signal. A possibility of increasing the bandwidth of the TRA focusing system is described in our co-pending patent application Ser. No. 12/036,531 filed Feb. 25, 2008 and entitled “Broadband Ultrasonic Probe”, which is incorporated herein by reference in its entirety. This possibility is based either on the use of broadband piezotransducers with variable thickness or on the simultaneous use of several transducers having different resonance frequencies. Generation of short pulses as well as a signal with arbitrary predetermined waveforms requires not only a wide spectral bandwidth of the focusing system but also sufficient flatness of that spectrum.
There is a need therefore for acoustic focusing methods which combine the advantages of the broad frequency band and the flatness of the frequency bandwidth. The invention described hereinafter meets such a need.